Dermal wound healing is a natural process which occurs whenever there is a break in the surface of the skin. An orderly, progressive process begins to take place in several stages, commencing with an inflammatory response necessary for natural sterilization of the area, clotting, and preparation of the wound for its natural progress toward closure.
The second stage of wound repair primarily involves migration of necessary cells to the wound site. These cells, which also appear in an ordered sequence during the second stage of healing, include key white cells, followed by polymorphonuclear leukocytes and macrophages--all essential to initiation of the repair process. These cells serve to clean up the wound and to deliver an endogenous mixture of growth factors to the wound site. In addition to white cells, fibroblasts from the wound periphery migrate to and into the site to deposit collagen, glycosaminoglycans, and other extracellular matrix components, providing the basis for granulation tissue formation. These macromolecules, in conjunction with capillary endothelial cells and new blood vessels, form the new tissue required to repair the defect.
The third stage of dermal wound healing involves proliferation of the cell types mentioned above together with subsequent increases in the amount of granulation tissue and coverage of the area with new epithelium.
Throughout the healing process, wounds produce a variety of fluids, generally known as wound exudate. The composition and quantity of this exudate may differ by wound type and with each stage of the wound's healing process. Exudate may vary--consisting primarily of blood and serous fluids to highly concentrated protein solutions. In general, wound exudate provides the necessary source of growth factors, nutrients and cells that are essential to the wound repair process. However, as with many biological processes, the amount of wound exudate must be controlled because over-production can provide a media for bacterial proliferation or lead to maceration of the surrounding healthy tissue. Conversely, inadequate types or amounts of wound exudate, or drying of the wound fluid, will lead to scab formation and impaired wound healing.
The control of wound exudate, referred to as moisture control, has become a major focus in current wound dressing design. The major categories of moist or synthetic wound dressings currently in use are thin films, hydrocolloids, hydrogels, and alginates. All of these various types of dressings control wound exudate through combinations of absorption, moisturization, and moisture permeability. The overall aim of moist wound dressings is to maintain the appropriate balance of moisture in the wound environment, which is essential to allow the natural wound healing process to occur.
Wound dressings which maintain a moist wound environment range from gels to thin films, foams, alginates, and hydrocolloids (see, e.g., Szycher et al., J. Biomater. Appl. 7:142-213, 1992). Thin film wound dressings consist of synthetic polymeric films which have one side coated with a pressure sensitive adhesive. This type of wound dressing has little or no absorptive ability and manages moisture primarily by the moisture vapor permeability of the polymeric film. Pooling and concentration of wound exudate under these dressings is commonly observed.
Hydrocolloids is a term commonly used to describe a family of wound management products consisting of gel forming agents combined with elastomers and adhesives attached to a polyurethane film or foam. This type of dressing usually has poor moisture vapor permeability, but some absorptive capability. These dressings typically absorb wound exudate by becoming thicker and forming a gelatinous material over the wound which has to be washed from the wound at each dressing change.
Hydrogel wound dressings are sheet dressings often supplied with an impermeable polymeric backing sheet. The presence of the backing sheet prevents the partially hydrated hydrogel from dehydrating and drying onto the wound bed. The hydrogels are similar to the hydrocolloids in their ability to absorb and manage wound exudate. Generally, hydrogel dressings do not dissolve into the wound bed as do hydrocolloids.
Foam dressings are generally manufactured from polymeric materials such as hydrophylic polyurethane. These foams contain open cells facing the wound surface which serve to absorb wound exudate by a sponge type mechanism. As with all sponges, the absorbed exudate is not tightly bound in the foam and a certain amount can be "squeezed" from the dressing upon removal.
Alginates are highly absorbent wound dressings composed of a variety of alginic acids extracted from certain species of seaweeds. The addition of a calcium salt during processing allows the formation of insoluble fibers which can be woven or formed into wound dressings of various sizes and shapes. During use, alginate wound dressing typically swell and form a soft gel in the wound bed. This gel must be washed from the wound bed at each dressing change.
In addition to the above, EP-A1-0 171 268 discloses a dressing for use in deep wounds which comprises small pieces of absorbent polyurethane foam contained within a porous bag or sachet. In a similar manner, EP-A1-0 575 090 discloses products suitable for absorbing wound exudate comprising particles of alginate or alginic acid enclosed in perforated bags.
Despite the wide variety of wound dressings available, chronic wounds are still a serious medical situation and there is a need for advanced wound care dressings which manage the moist wound environment. This includes the need for moisture controlling and absorbing wound dressings which do not leave a residue in the wound bed and which retain fluid when removed. The present invention fulfills these needs and provides further related advantages.